1. Field of the Invention
The present invention relates to a safety hypodermis syringe and, more particularly, to such a safety hypodermic syringe, which enables the needle cannula holder to be easily installed.
2. Description of Related Art
In a regular safety hypodermic syringe, the plunger has a hook at the front side, which is forced into engagement with the needle cannula holder after the service of the safety hypodermic syringe, enabling the needle cannula holder and the needle cannula to be pulled backwards with the plunger and received inside the barrel. According to conventional designs, the needle cannula holder is press-fitted into the inside of the front section of the barrel. The outer diameter of the needle cannula holder and the inner diameter of the front section of the barrel must match perfectly using the radial constraint force, so that the needle cannula holder can be positively positioned in the front section of the barrel. When inserting the needle cannula holder into the front section of the barrel, much force should be applied to the needle cannula holder, and the parts of the safety hypodermic syringe tend to be damaged when forcing the needle cannula holder into the front section of the barrel.
According to conventional safety hypodermic syringe fabrication methods, the needle cannula holder and the barrel are respectively molded from plastics by a respective multi-cavity injection-mold. Because the injection environment (location of running gate, injection temperature, cooling speed, and etc.) of each cavity in a multi-cavity injection-mold differs, it is difficult to control the precision size of every finished needle cannula holder and barrel. Because dimensional tolerance is inevitably, it is difficult to achieve perfect matching between the inner diameter of the barrel and the outer diameter of the needle cannula holder. Loose fitness between the outer diameter of the needle cannula holder and the inner diameter of the barrel may cause the needle cannula holder easily loosened from the barrel. Excessively tight fitness between the outer diameter of the needle cannula holder and the inner diameter of the barrel may cause the needle cannula holder unable to be pulled backwards with the plunger.
Therefore, it is desirable to provide an improved hypodermic syringe to mitigate and/or obviate the aforementioned problems.
The main object of the present invention is to provide a safety hypodermic syringe, which uses axial constraint force, instead of radial constraint force, to hold the needle cannula holder firmly in the barrel. It is another object of the present invention to increase the productivity of a safety hypodermic syringe. It is still another object of the present invention to provide a safety hypodermic syringe, which enables the needle cannula holder and the needle cannula to be easily pulled backwards and received inside the barrel after the service of the safety hypodermic syringe.
To achieve the objects, the safety hypodermic syringe of the present invention comprises a barrel and a needle cannula holder fastened to the barrel and holding a needle cannula. The barrel has a stop device suspended inside the front section of the barrel, and at lease two connecting ribs connected between the inside wall of the barrel and the stop device. The needle cannula holder comprises a front extension hub holding a needle cannula, a plurality of locating blocks protruded from the periphery of the front extension hub and stopped at the front side of the stop device, and a shoulder stopped at the rear side of the stop device. Therefore, the needle cannula holder is constrained by an axial force exerted from the stop device and connecting ribs of the barrel. After the service of the safety hypodermic syringe, the needle cannula holder is pushed more forwards to force the shoulder against the connecting ribs and to break the connecting ribs separate, for enabling the needle cannula holder and the needle cannula to be further pulled backwards into the inside of the barrel.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.